Fewer than six koalas were moved to a small island in Victoria in the 1920s to save them from hunters. A century later, scientists ran their genomes and found a result they did not expect

For decades, conservation biology has operated on a straightforward assumption. When a species suffers a dramatic population crash, its genetic future becomes increasingly bleak.A new Australian koala study, however, is starting to challenge this assumption.When scientists analysed the genomes of hundreds of koalas, they found that some populations thought to be genetically vulnerable for a long time appear to be showing signs of genetic recovery, even if they descended from a tiny group of survivors. The findings, published in Science, suggest that measuring genetic diversity may not always paint an accurate picture of extinction risk.The researchers surveyed koala populations across Australia, including in Victoria, where they had a direct link to the history of the country's largest wildlife rescue effort.The koalas on the brinkKoalas had, by the turn of the 20th century, been devastated by hunting for their fur, alongside rampant habitat destruction. Historical records show that southern Australian populations were pushed to the brink during the late 1800s and early 1900s.According to a study published in Heredity, available through Nature, some of Victoria's surviving koalas were rebuilt from small groups of founders. Two to three animals possibly established a population on French Island in the 1920s, while other island populations may have had as few as a handful of animals.For years, the southern populations were a warning of genetically compromised numbers. Severe bottlenecks were assumed to have produced a lack of genetic diversity, making these populations susceptible to inbreeding and accumulation of detrimental genes. The assumption made logical sense: genetic diversity is often seen as one of the strongest indicators of a species' ability to survive in the long run. The resultsThe widespread Black Summer fires of 2019-20 ignited a larger investigation into the genetic health of koalas. The researchers, comprising scientists from the University of Sydney and the conservation organisation Cesar Australia, successfully sequenced 418 complete koala genomes from across New South Wales, Queensland, and Victoria. The results, described in Science, did not necessarily align with previous assumptions.Although the northern populations generally carried more genetic diversity, they also exhibited higher genetic loads (more potentially detrimental mutations) and showed signs of reduced effective population size.The Victorian populations, which had experienced extremely severe historical bottlenecks, however, appear to be moving in the opposite direction. The analysis of their genes indicated they were expanding in size and gaining new genetic combinations through natural genetic reshuffling in recombination during reproduction. These processes appear to be associated with a reduced burden of deleterious genes and greater adaptive potential.Dr. Collin Ahrens, principal research scientist at Cesar Australia and co-author on the study, said the findings represent the "early signs of genetic regeneration" in genetically compromised populations.The significance of the findingsThe findings do not say that low genetic diversity suddenly no longer matters, but rather, conservationists may need to observe population dynamics over time, as opposed to relying solely on single measurements.Dr. Andrew Weeks, Director of Cesar Australia, argued that the study challenges the view that conservationists should simply consult a scorecard of genetic diversity in deciding extinction risks. Trends and evolutionary processes in a population may equally be important indicators of resilience.The implications extend far beyond koalas. A number of species worldwide have been severely impacted by population bottlenecks due to hunting, habitat fragmentation, disease, or climate change, and conservation efforts often rely on assessing the genetic health of these populations to guide their decisions.However, the researchers warned against using only static indicators alone for decision making, as it could be misleading and misinterpret which populations are truly at risk.The koala's uncertain futureThe timing of these findings couldn't be more fitting, with koalas facing divergent futures across the continent. Northern koalas continue to be battered by the pressures of habitat loss, diseases, road accidents, and extreme fire events. They are now listed as Endangered across the majority of Queensland, New South Wales, and the Australian Capital Territory.Contrastingly, pockets of southern Australia are currently overpopulated with koalas, posing challenges of their own.The study does not mean the koala's days are over, but it does suggest that extinction may not always follow a straight line, and populations can respond and recover from devastating loss.The implications for conservation biologists are clear: genetic diversity may still matter, but it's only one piece of the pie.A genetically vulnerable population on paper may actually be evolving in the right direction. The relocated handful of koalas that were moved onto an island over a century ago in an attempt to prevent them from going extinct are now helping scientists reassess how genetic risk is measured in conservation biology.